The fact that we even get to see these minerals is remarkable, when you consider the feats of nature that have to coincide to bring them anywhere near our reach. And let alone the human endeavour and ingenuity needed to find and mine them.
The story of any diamond begins deep within the Earth, and at least a billion years ago.
A matter of time
Diamonds form from carbon that has been subjected to extremely high temperatures and pressures over long periods of time. These conditions exist only at depths of 150 to 200km beneath the surface of the Earth, in a part of the planet known as the mantle.
The diamonds that we mine today typically formed around a billion years ago. To put that age into context, it means they are more than twice as old as life on dry land. The first terrestrial plants appeared in a period of geological time known as the Silurian, which began 443 million years ago, long after the diamonds you see in a jewellery store window were crystallising.
Some are considerably older – at the Diavik mine in Canada, diamonds can be more than 3.3 billion years old. The Earth is thought to be 4.5 to 4.6 billion years old, which means one of these diamonds could be three-quarters as old as our planet itself.
So why do we find diamonds near the surface, if they formed deep in the bowels of the Earth? It’s a dramatic part of their story, which geologists have only understood well in the last 50 years.
The rocks in which we find diamonds today were not the rocks in which they crystallised. Down in the mantle, beneath the level of diamond formation, molten rock – known as magma – formed. This magma forced itself upwards, melting through the diamond-bearing rock. Being resistant to high temperatures, the diamonds were preserved, and carried towards the surface.
Meanwhile, volatile compounds such as water and carbon dioxide dissolve in the magma, and as the pressure reduced near the surface, these volatiles turned into gas. This gas forced some of the magma to burst through the crust in explosive eruptions, while the rest was left to solidify in vertical pipe-like structures underground.
No human has ever witnessed this kind of eruption. Although these blasts occurred far more recently than the diamonds originally formed, they still typically happened tens of millions of years ago.